1. Field of the Invention
The present invention relates to a multi-antenna apparatus and a mobile device, and more particularly, it relates to a multi-antenna apparatus and a mobile device each comprising a plurality of antenna elements.
2. Description of the Background Art
A multi-antenna apparatus comprising a plurality of antenna elements is known in general, as disclosed in Japanese Patent Laying-Open No. 2007-97167, for example.
The aforementioned Japanese Patent Laying-Open No. 2007-97167 discloses a MIMO array antenna (multi-antenna apparatus) comprising two antenna elements each constituted by a monopole antenna and an isolation element arranged between the two antenna elements for reducing a mutual coupling between the two antenna elements. In this MIMO array antenna, the two antenna elements are spaced apart a distance of one half of a wavelength λ of the corresponding radio wave from each other and the isolation element is arranged at a position separated from each of the two antenna elements by a distance of λ/4, thereby resonating the isolation element to reduce the mutual coupling between the antenna elements each constituted by a monopole antenna.
An antenna element constituted by a loop antenna (looped antenna) having characteristics different from those of a linear antenna such as a monopole antenna is known in general, as disclosed in Japanese Patent Laying-Open No. 2006-93977, for example.
The aforementioned Japanese Patent Laying-Open No. 2006-93977 discloses a loop antenna unit comprising a loop antenna having a pair of open edges on both ends thereof and a metal member arranged in the vicinity of the loop antenna, electrically connected to one of the pair of open edges of the loop antenna.
In relation to the MIMO array antenna (multi-antenna apparatus) according to the aforementioned Japanese Patent Laying-Open No. 2007-97167, no structure of reducing a mutual coupling between antenna elements each constituted by a loop antenna is described although the mutual coupling between the antenna elements each constituted by a monopole antenna can be reduced. As hereinabove described, a loop antenna has characteristics different from those of a linear antenna such as a monopole antenna, and hence it is conceivably impossible to apply an art based on the premise of the monopole antenna according to the aforementioned Japanese Patent Laying-Open No. 2007-97167 to the loop antenna described in the aforementioned Japanese Patent Laying-Open No. 2006-93977.
Therefore, in general, it is difficult to reduce a mutual coupling between antenna elements each constituted by a loop antenna when a multi-antenna apparatus is constituted by loop antennas. Consequently, it is disadvantageously difficult to downsize the multi-antenna apparatus including the loop antennas (looped antennas).